This invention relates to plant quinolate phosphoribosyl transferase (QPRTase) and to DNA encoding this enzyme. In particular, this invention relates to the use of DNA encoding quinolate phosphoribosyl transferase to produce transgenic plants having genetically altered nicotine levels, and the plants so produced.
The production of tobacco with decreased levels of nicotine is of interest, given concerns regarding the addictive nature of nicotine. Additionally, tobacco plants with extremely low levels of nicotine production, or no nicotine production, are attractive as recipients for transgenes expressing commercially valuable products such as pharmaceuticals, cosmetic components, or food additives. Various processes have been designed for the removal of nicotine from tobacco. However, most of these processes remove other ingredients from tobacco in addition to nicotine, thereby adversely affecting the tobacco. Classical crop breeding techniques have produced tobacco plants with lower levels of nicotine (approximately 8%) than that found in wild-type tobacco plants. Tobacco plants and tobacco having even further reductions in nicotine content are desirable.
One approach for reducing the level of a biological product is to reduce the amount of a required enzyme in the biosynthetic pathway leading to that product. Where the affected enzyme naturally occurs in a rate-limiting amount (relative to the other enzymes required in the pathway), any reduction in that enzyme""s abundance will decrease the production of the end product. If the amount of the enzyme is not normally rate limiting, its presence in a cell must be reduced to rate-limiting levels in order to diminish the pathway""s output. Conversely, if the naturally-occurring amount of enzyme is rate limiting, then any increase in the enzyme""s activity will result in an increase in the biosynthetic pathway""s end product.
Nicotine is formed primarily in the roots of the tobacco plant and is subsequently transported to the leaves, where it is stored (Tso, Physiology and Biochemistry of Tobacco Plants, pp. 233-34, Dowden, Hutchinson and Ross, Stroudsburg, Pa. (1972)). An obligatory step in nicotine biosynthesis is the formation of nicotinic acid from quinolinic acid, which step is catalyzed by the enzyme quinoline phosphoribosyl transferase (xe2x80x9cQPRTasexe2x80x9d). QPRTase appears to be a rate-limiting enzyme in the pathway supplying nicotinic acid for nicotine synthesis in tobacco. See, e.g., Feth et al., xe2x80x9cRegulation in Tobacco Callus of Enzyme Activities of the Nicotine Pathwayxe2x80x9d, Planta, 168, pp. 402-07 (1986); Wagner et al., xe2x80x9cThe Regulation of Enzyme Activities of the Nicotine Pathway in Tobaccoxe2x80x9d, Physiol. Plant., 68, pp. 667-72 (1986). The modification of nicotine levels in tobacco plants by antisense regulation of putrescence methyl transferase (PMTase) expression is proposed in U.S. Pat. Nos. 5,369,023 and 5,260,205 to Nakatani and Malik. PCT application WO 94/28142 to Wahad and Malik 30 describes DNA encoding PMT and the use of sense and antisense PMT constructs.
A first aspect of the present invention is an isolated DNA molecule comprising SEQ ID NO:1; DNA sequences which encode an enzyme having SEQ ID NO:2; DNA sequences which hybridize to such DNA and which encode a quinolate phosphoribosyl transferase enzyme; and DNA sequences which differ from the above DNA due to the degeneracy of the genetic code. A peptide encoded by such DNA is a further aspect of the invention.
A further aspect of the present invention is a DNA construct comprising a promoter operable in a plant cell and a DNA segment encoding a quinolate phosphoribosyl transferase enzyme positioned downstream from the promoter and operatively associated therewith. The DNA encoding the enzyme may be in the antisense or sense direction.
A further aspect of the present invention is a method of making transgenic plant cell having reduced quinolate phosphoribosyl transferase (QPRTase) expression, by providing a plant cell of a type known to express quinolate phosphoribosyl transferase; transforming the plant cell with an exogenous DNA construct comprising a promoter and DNA comprising a portion of a sequence encoding quinolate phosphoribosyl transferase mRNA.
A further aspect of the present invention is a transgenic plant of the species Nicotiana having reduced quinolate phosphoribosyl transferase (QPRTase) expression relative to a non-transformed control plant. The cells of such plants comprise a DNA construct which includes a segment of a DNA sequence that encodes a plant quinolate phosphoribosyl transferase mRNA.
A further aspect of the present invention is a method for reducing expression of a quinolate phosphoribosyl transferase gene in a plant cell by growing a plant cell transformed to contain exogenous DNA, where a transcribed strand of the exogenous DNA is complementary to quinolate phosphoribosyl transferase mRNA endogenous to the cell. Transcription of the complementary strand reduces expression of the endogenous quinolate phosphoribosyl gene.
A further aspect of the present invention is a method of producing a tobacco plant having decreased levels of nicotine in leaves of the tobacco plant by growing a tobacco plant with cells that comprise an exogenous DNA sequence, where a transcribed strand of the exogenous DNA sequence is complementary to endogenous quinolate phosphoribosyl transferase messenger RNA in the cells.
A further aspect of the present invention is a method of making a transgenic plant cell having increased quinolate phosphoribosyl transferase (QPRTase) expression, by transforming a plant cell known to express quinolate phosphoribosyl transferase with an exogenous DNA construct which comprises a DNA sequence encoding quinolate phosphoribosyl transferase.
A further aspect of the present invention is a transgenic Nicotiana plant having increased quinolate phosphoribosyl transferase (QPRTase) expression, where cells of the transgenic plant comprise an exogenous DNA sequence encoding a plant quinolate phosphoribosyl transferase.
A further aspect of the present invention is a method for increasing expression of a quinolate phosphoribosyl transferase gene in a plant cell, by growing a plant cell transformed to contain exogenous DNA encoding quinolate phosphoribosyl transferase.
A further aspect of the present invention is a method of producing a tobacco plant having increased levels of nicotine in the leaves, by growing a tobacco plant having cells that contain an exogenous DNA sequence that encodes quinolate phosphoribosyl transferase functional in the cells.